Electrolytic treatment of stainless steel having an oxidic film

ABSTRACT

Oxidic films on stainless steel and similar alloys formed by treatment of the alloys in aqueous solutions of sulfuric acid and pitting inhibiting oxidizing agents, are hardened by being treated as cathodes in aqueous electrolytes containing metal ion having a pH in excess of about 0.5 and within the range for the particular metal of the solution whereby oxide will form at the cathode rather than metal.

United States Patent [191 Skedgell et al.

[ 1 Nov. 26, 1974 ELECTROLYTIC TREATMENT OF STAINLESS STEEL HAVING AN ()XIDIC FILM Inventors: Anthony Northcott Skedgell,

Birmingham; Victor Albert Smith,

Warley; Thomas Ernest Evans, Solihull, all of England The International Nickel company, Inc., New York, NY.

Filed: Oct. 27, 1972 Appl. No.: 301,640

Assignee:

Foreign Application Priority Data Nov. 3, 1971 Great Britain 5'1164/72 US. Cl. 204/56 R, 204/35 N Int. Cl C23b 11/02,

Field of Search 204/41, 56 R, 35 N [56] References Cited UNITED STATES PATENTS 3,535,213 10/1971) Okada 204/41 Primary Examiner*R. L. Andrews 9 Claims, No Drawings ELECTROLYTIC TREATMENT OF STAINLESS STEEL HAVING AN OXIDIC FILM The present invention is concerned with hardening oxidic films formed on iron-base, chromiumcontaining, corrosion-resistant alloys, e.g., stainless steels, by treatment of said alloys in aqueous sulfuric acid solutions containing a pitting inhibiting oxidizing agent(advantageously chromic acid).

A known method of treating stainless steel and other chromium-containing alloys comprises two main steps, in the first of which an oxidic film is formed on the surface of the alloy by immersion of the alloy in an aqueous solution of chromic and sulfuric acids, with or without other constituents, and in the second of which the alloy bearing the film is subjected to electrolysis as the cathode in an electrolyte from which chromium can be deposited. This method is the'subject of US applications Ser. Nos. 1 14,357 and 252,459, filed in the name of Anthony C. Hart, in which it is explained that the electrolytic treatment is short, lasting for a period of time adequate to harden the film, but not so long that any chromium becomes visible on the surface as a white deposit.

in practice of the Hart process, there is an increasing tendency for chromium metal to be deposited from aqueous electrolytes containing chromic and sulfuric acids as the current density increases, and, at the comparatively low current density required to ensure that no chromium metal is deposited, it is difficult to harden the film completely. Our object in the present invention is to eliminate or at least to reduce this disadvantage.

We have found that the acidity of the electrolyte plays an important part in the hardening step, and that above a certain pH value in an electrolyte containing hexavalent chromium ions, no chromium metalis deposited. We have also found using aqueous solutionsof metal ion as electrolytes that above about a pHof 0.5 andwithin a restricted range of pH particular to each metal of the group set forth hereinafter, metal oxidic species will be deposited preferentially at a cathode and that these aqueous solutions of metal ions can be used as electrolytes in the electrolysis process to harden films on stainless steel in a manner similar to the use of aqueoussolutions containing hexavalent chromium.

It is an object of the invention to provide a novel pro cess for hardening oxidic films on stainless steel and similar alloys.

Another object of the present invention is to provide an advantageous electrolyte useful for hardening oxidic films on stainless steel and similar alloys.

Generally speaking, the present invention contemplates a process for hardening an oxidic film produced on an iron-base, chromium-containing, corrosionresistant alloy by treatment in an aqueous sulfuric acid bath containing a pitting inhibiting oxidizing agent comprising treating as the cathode the surface of such an alloy bearing such an oxidic film in an aqueous solution of metal from the group of aluminum, titanium, vanadium, chromium, molybdenum. tungsten, manganese, iron, cobalt, nickel, copper, zinc, tin and silicon, saidaqueous solution having a pH in excess of about 0.5 and within a specially selected range particular as to each metal-bearing ion so as to deposit at the cathode surface metal oxide, hydrated oxide or hydroxide in preference to metal and thereby increase the resistance of said oxide film to abrasion.

Generally speaking, if the pH of the solutions is too low, metal oxide will not be formed at the cathode, whereas if the pH of the solution is too high, the metalbearing ion will sometimes tend to hydrolyze in the bulk of the solution to form an oxidic precipitate.

According to one aspect of the invention, the composition of an electrolyte containing hexavalent chromium ions is formulated such as to bring the pH into the range in which chromium metal is not deposited. This can be done by the addition of sodium hydroxide or ammonium hydroxide to a chromic acid electrolyte.

Generally speaking, this aspect of the process of the present invention comprises cathodically treating the surface of an iron-base, chromium-containing, corrosion-resistant alloy bearing an oxidic film produced by treatment in an aqueous sulfuric acid bath containing a pitting inhibiting oxidizing agent in an aqueous electrolyte containing about 25 grams per liter (gpl) to about 750 gpl of hexavalent chromium specie measured as CrO and a hydroxide, carbonate or the like from the group consisting of ammonium, potassium and sodium hydroxide and carbonate in an amount sufficient to at least partly neutralize the chromiumcontaining anions and sufficient to maintain the bulk pH of the electrolyte in the range of about 3 to about 7.5. The cathodic treatment is carried out at a cathode current density of about 2 to about 40 amperes per square decimeter (A/dm) for about 2 to about 40 minutes while the electrolyte is held at a temperature of about 20C. to about 60C. Advantageously, the electrolyte contains at least about gpl of chromic-acid (measured as CrO A particularly useful standard electrolyte contains about 2.5 moles of CrO 250 grams) per liter and sufficient NaOH or NH OH (about 3.5 to 5.2 moles of NaOH) to bring the pH of the electrolyte to about 6.3 to about 7.5. Advantageously, the cathode current density during hardening is within the range of about 8 to about 12 A/dm.

The initial film formed by immersion of stainless steel in chromic and sulfuric acids probably consists of a hydrated iron and chromium-rich, spinel-type oxide and it seems that the hardening result from the entry of Cr,0 into the pores of this film. Chromium metal may also be deposited on the surface and if this happens it spoils the appearance of the film. Rendering the electrolyte less acidic appears to promote electrolytic conditions within the film which favor deposition of Cr,0, into the pores. However, this may be in a hardening electrolyte consisting of 2.5 molar (M) CrO and 0.01M (NH S O and operated at a current density of about l0.8 A/dm chromium metal is deposited and only a moderate amount of hardness is achieved. When NaOH is progressively added to the electrolyte, chromium metal continues to be deposited until the molar ratio NaOH: CrO exceeds 0.8. When the molar ratio is within the range 0.4 to 2.1 there is a striking increase ably adjusted within the range 1.4 to 2.1 to yield pH within the range 6.3 to 7.5 by the addition of from 3,5M to 5.2M NaOH.

Typical conditions for the hardening of blue films formed on 18/8 stainless steel are a cathodic current 5 current densities without the risk of metallic chromium density of 10.8 A/dm for about minutes while the deposition. Above pH 7.5 no effective hardening is ob electrolyte is at a temperature of about 40C. tained and below pH 3, metallic chromium forms on This aspect of the invention includes hexavalent the surface at high cathode current densities. chromium-containing electrolytes in which the harden- It has also been found that oxides other than chroing can be effected, these containing the reaction prodl0 mium can be utilized for hardening the film. To this end ucts of from 0.25M to 7.5M CrO together with basic the metal carrying the coating or colored film to be ions, e.g., sodium or ammonium ions, adequate to bring hardened is immersed in an aqueous solution of salts of, the pH into the range of 3 to 7.5. Such an electrolyte for example, aluminum, titanium, vanadium, molybdecan be made by adding chromic acid to a solution of so num, tungsten, manganese, iron, cobalt, nickel, copper, um Chr ammonium Chromate, Or y adding 5 zinc, tin and silicon and made the cathode in the solusodium or ammonium hydroxide to a solution of chrotion. Subsequently, cathodic electrolysis is carried out mic acid. until the film is hardened.

The Invention presents the thre ad antag of The selection of a suitable hardening solution emcreasing the hardness of the films, avoiding the deposil i a tal salt t il the hoi of a alt of a tion Of Chromium metal and enab lng t Current metal which has an oxide insoluble in water and which sity to be in reas d during t a d g P preferably forms a neutral or slightly acidic solution.

In Order to g those Skllled the an a better under- Either the solution of such a salt must be capable of de- Stahdlhg and appreclahoh of thls p of the positing the oxide if the pH value of the solution is the following examples are g raised, or it must form an oxide by cathodic reduction of a soluble species in the solution or it must form an EXAMPLES oxide by a combination of both these effects. The color A genes hardehlhg baths were Set p l of the oxide deposited has little or no effect on the mg composmohs m moles per llter (mm as Set forth m color of the film, although the color of the film is usu- Table I. Baths designated by letters are outside the n Slightly advanced in shade ambit of the present invention, whereas those marked ExamP|eS f mirror fi i h Type 304 staimess steel by numbers are examples of baths within the ambit of plate given a blue m coating b immersion i a h the Present h mic and sulfuric acids solution, and hardened by cathodic electrolysis in an electrolyte according to the in- TABLE I vention will now be described. In these examples, the

B, h C O NH so 0 N. OH c t experimental variables, apart from the solution, include Designs "l/ii (iii/i) I (fll/ D eh fil y H current density, time of process, pl-l value and temperatlun (A/dm) ture, and the effects of the hardening treatment are to harden the film and slightly to advance the color of the A 2.5 0.01 0 9.7 B 2.5 um 1 11,6 film in shade. Undesirable effects during hardening are f i metal deposition, oxide deposition on top of the film 3 5:}; m 1 and too great or uneven color advancement. Hardness 3 3 (WI 5 jof the film is measured in the examples by a standard D rub-test which consists of rubbing the film surface emit-mimic, ne ative H .iuc with a pencil-type eraser, preferably Remington,

loaded with a 400 gram weight. For example, an unl I l d hardened film would fail in one or two rubs whereas a Type Slam ess.st.ee i l g g T a; film hardened according to the invention may withous so comammg C romlc 9 stand 100 to 150 rubs or more before failure. to produce on the surface thereof an OXldlC film having a blue color. An arbitrary test for abrasion resistance EXAMPLE 4 compris ng rubbing with a green rubber eraser was set Potassium permanganate: in an aqueous Solution up g 3; foulnd that i g igs glsg g s z agf was employed as the hardening electrolyte. The mow e W1 y one or p hardening mechanism makes use of the fact thatMn than hardened in the baths set froth in Table l by treats m the form of the MnO anion is stable in solution mg them cathodically at the cathodic current densities th t l alkal. H ran e is thou h that when indicating in Table l for 15 minutes with the results l i a d z d i g Within the being obtained as set forth in Table H. n 4 18 re a e ca 9 e pores of the color-film, Mn 15 produced and precipi- TABLE ll tated as the insoluble Mnoz, Possibly hydrated. Test conditions were 0.4M KMnO4 (63 gpl) solution, Bath Rubs for Film Removal Over Film Deposit pH 8.5 at room temperature. Results of the tests are Designation (average of 3 trials) (if any) set forth in Table [IL A 35 Cr metal B 50 Cr metal TABLE lll C 55 Cr metal I no 65 2 65 Current Time Hardness 3 8t) Density (in (no. of rubs Remarks D 25 (r(OH) (A/dm) sec.) to failure) 0.05 30 30 Slight color advancement to Tables I and II show that when the pH of a hardening solution containing hexavalent chromium is between 3 and 7.5 advantageous improvements of abrasion resistnace of oxidic films can be obtained at high cathodic late (pale) blue Conclusions to be drawn from the data set forth in Table III and other observations are that use of cathodic treatment in potassium permanganate solution results in (a) moderate hardening increase; (b) improved resistance to finger marking; and considerable color advancement.

EXAMPLE Nickel nitrate in an aqueous solution was employed as the hardening electrolyte. It is thought that unlike the KMnO, or CrO hardening process no electrochemical reduction of metal ion species takes place. The main reduction at the cathode is hydrogen ion discharge, H e k H This causes the pH values within the pores of the color film to rise, the solubility product of the metal-hydroxide is exceeded and nickel hydroxide is precipitated within the pores. 0.5M (91 gpl) Ni(NO was used. Results of tests using nickel nitrate as the electrolyte are set forth in Table IV.

TABLE IV Current Density tA/dm l Hardness (no. of rubs to failure) Time (in sec.)

P Remarks Ni(OH) deposition within pores as shown hy slight color advancement. Also some N|(OH) deposited on top of color film Ni(OH deposition only within pores of color film. Slight color advancement Slight color advancement.

Ni(0H deposition only wihtin pores of film Slight color advancement. Ni(OH) deposition only within pores of film The last three experiments in Table IV were repeated using Ni(NO solutions with pH values adjusted to 5.7 and 4.7 with very similar results. Raising the temperature to 40C. and 60C. did not cause any further improvement.

The data in Table IV shows that reasonable hardening is possible in Ni(NO solutions. As long as the current and time are restricted as shown in Table IV, there are no problems of changes in appearance of the specimens.

EXAMPLE 6 Zinc nitrate in an aqueous solution was employed as the hardening electrolyte. It is thought that the hardening mechanism is the same as for the Ni(NO process of Example 5. Test conditions were 0.5M (95 gpl) Zn(NO at room temperature. Results are set forth in Table V.

TABLE V Time (in sec.)

Hardness (no. of rubs to failure) Current Density (A/clm pH Remarks Zn(OH), deposition within pores as shown by slight color advancement. Also some Zn(OH) deposited on top of color film Zn(OH) deposition only within pores of color film. Slight color advancement Slight color advancement. Zn(0l-l), deposited only within pores of color Zn(OH) deposition in pores and on top of color film The Zn(NO solution gave some hardening although the results were not as good as in the Ni(NO solution of Example 5. It appears necessary for the current and time to be reduced to avoid loose oxide/hydroxide deposit on top of the color film and this limits the degree of hardening that can be achieved. Reducing the pH to 1.5 shifted the acceptable current density and time to higher levels, but no increase in hardness resulted.

EXAMPLE 7 Aluminum sulfate in an aqueous solution was employed as the hardening electrolyte. It is thought that the hardening mechanism is the same asfor Examples 5 and 6. Test conditions were 0.5M (171 gpl)'Al (SO9 at room temperature. Results are set forth in Table VI.

TABLE VI Current Time Hardness Density (in pH (no. of rubs Remarks (A/dm) sec.) to failure) 0.4 60 2 20 Slight color advancement 4.0 15 2 I00 Moderate color advancement. particularly around edges 4.0 15 l Moderate color advancement, particularly around edges The data in Table VI show that in Al S0 solutions it is possible to get reasonably good hardening without too much color advancement or oxidedeposition on top of the color film.

When employed in this specification and claims, the term iron-base, corrosion-resistant, chromiumcontaining alloy includes stainless steels and other iron-containing alloys which also contain greater than about 1 l percent and up to about 30 percent by weight of chromium. Stainless steels can be ferritic or austenitic and usually contain about 13 percent to about 25 percent (by weight) chromium.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for hardening an oxidic film on ironbase, chromium-containing, corrosion-resistant alloy formed by treatment of said alloy in an aqueous solution of sulfuric acid and chromic acid which process comprises treating said alloy having said film on the surface thereof as a cathode in an aqueous solution having a pH in excess of about 0.5, containing in ionic form metal from the group consisting of aluminum, titanium, vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, copper, zinc, tin and silicon and having the pH regulated within a special range for each type of ion so that oxide will be deposited at said cathodic surface in preference to metal.

2. A process as in claim 1 wherein the iron-base,

chromium-containing, corrosion-resistant alloy is stainless steel.

3. A process as in claim 2 wherein the aqueous solution contains about 0.25 to about 7.5 moles per liter of CrO and sufficient base to maintain the pH of the solution in the range of about 3 to about 7.5.

4. A process as in claim 3 wherein the aqueous solution has a pH of about 6.3 to about 7.5.

5. A process as in claim 1 wherein the metal ion in aqueous solution is hexavalent chromium and the pH is in the range of about 3 to about 7.5.

6. A process as in claim 1 wherein the metal ion in aqueous solution is trivalent aluminum.

7. A process as in claim 1 wherein the metal ion in solution is heptavalent manganese.

8. A process as in claim 1 wherein the metal ion in solution is divalent nickel.

9. A process as in claim 1 wherein the metal ion in solution is divalent zinc. 

1. A PROCESS FOR HARDENING AN OXIDIC FILM ON IRON-BASE, CHROMIUM-CONTAINING, CORROSION-RESISTANT ALLOY FORMED BY TREATMENT OF SAID ALLOY IN AN AQUEOUS SOLUTION OF SULFURIC ACID AND CHROMIC ACID WHICH PROCESS COMPRISES TREATING SAID ALLOY HAVING SAID FILM ON THE SURFACE THEREOF AS A CATHODE IN AN AQUEOUS SOLUTION HAVING A PH IN EXCESS OF ABOUT 0.5, CONTAINING IN IONIC FORM METAL FROM THE GROUP CONSISTING OF ALUMINUM, TITANIUM, VANADIUM, CHROMIUM, MOLYBDENUM, TUNGSTEN, MANGANESE, IRON, COBALT, NICKEL, COPPER, ZINC, TIN AND SILICON AND HAVING THE PH REGULATED WITHIN A SPECIAL RANGE FOR EACH TYPE OF ION SO THAT OXIDE WILL BE DEPOSITED AT SAID CAHODIC SURFACE IN PREFERENCE TO METAL.
 2. A process as in claim 1 wherein the iron-base, chromium-containing, corrosion-resistant alloy is stainless steel.
 3. A process as in claim 2 wherein the aqueous solution contains about 0.25 to about 7.5 moles per liter of CrO3, and sufficient base to maintain the pH of the solution in the range of about 3 to about 7.5.
 4. A process as in claim 3 wherein the aqueous solution has a pH of about 6.3 to about 7.5.
 5. A process as in claim 1 wherein the metal ion in aqueous solution is hexavalent chromium and the pH is in the range of about 3 to about 7.5.
 6. A process as in claim 1 wherein the metal ion in aqueous solution is trivalent aluminum.
 7. A process as in claim 1 wherein the metal ion in solution is heptavalent manganese.
 8. A process as in claim 1 wherein the metal ion in solution is divalent nickel.
 9. A process as in claim 1 wherein the metal ion in solution is divalent zinc. 